So what does it mean to be dead?

Still, if anyone asked me what it meant for a Christian to be dead because of sin, while the Spirit is life because of righteousness, I'm not sure I'd know. It does sound like a very definite thing that happens to everyone (if Christ is in you, the body IS dead), and also passive (though, a couple verses down it clarifies...if by the Spirit we put to death the deeds of the body....)My best guess is that it has to do with motivations again, and also perhaps that it is a slow process. That's the only way it makes sense to me.

That is, your body being dead because of sin means that when you have Christ, when you know what true righteousness is, you can no longer trust and follow your motivations apart from him, because you now know that every single one of them is tainted and corrupted. So maybe you were always dying, but Christ being in you made you realize it. And then dying to sin, dying to the law... I like the example Paul uses elsewhere about dying to the law like a husband's death frees a woman to marry again.

Here's another passage, also by Paul, from 2 Corinthians 4:

We always carry around in our body the death of Jesus, so that the life of Jesus may also be revealed in our body. For we who are alive are always being given over to death for Jesus' sake, so that his life may be revealed in our mortal body. So then, death is at work in us, but life is at work in you. It is written: "I believed; therefore I have spoken." With that same spirit of faith we also believe and therefore speak, because we know that the one who raised the Lord Jesus from the dead will also raise us with Jesus and present us with you in his presence. All this is for your benefit, so that the grace that is reaching more and more people may cause thanksgiving to overflow to the glory of God.

In this case the death of the body clearly means joining the suffering of Jesus... in the verses before, he was talking about the sufferings that they were undergoing to tell people about Jesus. This is such a lovely picture of the kingdom, and I think a good parallel with Romans 8. There's the same idea of a continual death of the body (through continually denying it comfort, fellowship, etc, for the sake of doing what Jesus wants them to do), and because the death is for the sake of Jesus, the life that we live, even in our mortal bodies full of sin, is Jesus' life... that is, it is revealed in us. And so the life of Jesus overflows from our bodies full of death to other people. So, as Romans 8 says, if we do have Christ the body is dead because of sin (because we deny the body's demands, since we follow Christ now, so by the Spirit we put to death the body's demands). And so the life we do live is Christ's.